1. Field of the Invention
The present invention relates to methods and apparatus for soldering, and more particularly concerns fluxless soldering of high density leads of an electronic circuit chip.
2. Description of Related Art
Connection of electrical leads to integrated circuit or electrical microchip elements is accomplished by various procedures, including ultrasonic bonding and soldering. In ultrasonic bonding an end of a wire, generally of a circular cross section, is pressed against a pad to which it is to be bonded by the tip of a bonding tool that is connected to an ultrasonic transducer. Energization of the transducer while the bonding tool tip presses the wire against its pad deforms the wire under pressure and vibration, thereby providing an electrical connection. Soldering of leads is sometimes preferred for repairability and the ability to make many solder connections in one step. In conventional soldering, an end of the lead is held against a substrate pad to which it is to be bonded, solder is applied to the bonding area, and the parts are heated to cause the solder to flow around or between the parts, thereby to firmly connect the parts both mechanically and electrically upon cooling and solidification of the solder. Frequently the leads to be soldered are "pre-tinned", that is, pre-coated with a thin layer of solder, as by dipping, silk screening or plating, and a solder flux is applied to the bonded area to ensure the making of a proper bond.
In making solder connections at the outer ends of a large nubber of electrical leads that may radiate in many directions from a central electronic chip a single pressure foot or "thermode" is employed in a gang bonding operation to press ends of all the wires against their respective pads, and to simultaneously apply heat for melting the solder of the pre-tinned lead ends. The thermode is provided with a planar pressure surface that has a high precision of flatness. However, the substrate to which leads are to be bonded carries a number of pads that may be positioned at slightly different elevations, because cause of such factors as variation in thickness of pads and adhesive used to hold the pads, and irregular surfaces of the substrate which carries the pads. In other words, the surface of the pads to which the outer ends of the radiating wires that are to be bonded do not all lie in the same plane with any degree of precision. Accordingly, as the unitary thermode simultaneously presses the ends of as many as two to three hundred wires against the pads, some wire ends are not pressed or are not adequately pressed against the pads to which they are to be bonded, resulting in either a poor bond or total absence of bond at some wire end.
Flux, now widely used in soldering, introduces contaminants, is difficult to apply with precision, and, if excess flux is used, the appearance of the component may be degraded. There is always some residual flux remaining even after very careful cleaning of the parts.
Accordingly, it is an object of the present invention to provide for the bonding of electrical leads while avoiding or minimizing above-mentioned problems.